I think their mindset is just human must stick together no matter it take. Thats why she didnt even realise he can switch side.Such passionate words from a naked man wearing a condom on his head...
Though in all seriousness, in what world did the human girls actually think he'd listen to them at this point? Sure, he's put up with their non-stop abuse so far thanks to his own idiocy and them realizing that he's their only real hope, but they practically abuse him every other page when there isn't a fight happening.
Why would he side with them? Especially now that their cruelty towards the demon girls is shown so openly?
Nobody in this manga is good(megane healer maybe only exception), and all are doing whatever the h*** they want for their own ends. The witch and the knight have also probably killed demons for no other reason other than they are demons. At the very least, the MC has a point that they treated him well and protected him, so if we are to put ourselves in his shoes, then yeah, they deserve the MCs protection and sympathy.Remember that in chapter 7, these 3 specific demons killed 2 random villagers for no reason other than the fact they simply wanted to. One of which was legitimately scared and overtaken by them against his will. The manga plays it for laughs but they still killed 2 people.
Now, because they said they loved the MC, we're supposed to feel bad and side with him and ignore that these things have shown they have a history of malicious intent.
Who exactly am I supposed to be rooting for? Because quite frankly everyone seems to be a terrible person in this series.